Hey, fighting game fans, hope you didn't delete your save files from the pre-Ultimate version of Marvel vs. Capcom 3. Because playing as the big bad from MvC3 in UMvC3 is significantly easier if you've still got that data on your console's hard drives.
According to a post over on Capcom Unity, all you need to do to play as the Devourer of Worlds in one of UMvC3's special modes is have save data from the original game. Otherwise, you'll need to farm your way to 30,000PP (player points) to unlock the Big G. That's a lot of grinding just to play an add-on, no?
How to unlock Galactus mode in Ultimate MvC3 [Capcom Unity]